This invention generally relates to the fabrication of thin film capacitors such as those in dynamic random access memory devices.
Without limiting the scope of the invention, its background is described in connection with current methods of forming thin film capacitors, as an example.
Heretofore, in this field, thin film capacitors such as those in DRAMs have used SiO2 or Si3N4 as the dielectric material. As the destiny of intergrated circuits (number of devices per square centimeter) increases, the capacitors which store electrical charge in each DRAM memory device must decrease in size while maintaining approximately the same capacitance. Referring to the following equation, C is the capacitance of a planar capacitor, xcex5 is the dielectric constant, xcex50 is the dielectric permittivity of free space, A is the area, and d is the thickness of the dielectric.   C  =                    εε        0            ⁢      A        d  
It is seen that the capacitance is directly proportional to the dielectric constant and inversely proportional to the thickness. Therefore, to build smaller capacitors while maintaining the same capacitance, one must increase xcex5 and/or decrease the dielectric thickness.
One method of allowing a decrease in capacitor area is to make use of materials with a much higher dielectric constant than SiO2 or Si3N4. The dielectric constant for both of these materials is less than ten. An important class of high dielectric constant materials is the perovskites (e.g. BaTiO3 and SrTiO3). The dielectric constants of these materials may be as high as 10,000 when they are fabricated as bulk ceramics. To be useful in the manufacture of miniature capacitors, these materials must be formed in thin films in a way that preserves their high dielectric constants relative to that of SiO2.
This invention is a method for increasing the dielectric constant of perovskite materials. The perovskite materials are: (1) any material exhibiting the well known perovskite crystal structure, typified by CaTi03; and (2) compounds with structures which can be derived from the ideal cubic perovskite structure by way of small lattice distortions or omission of some atoms. Many perovskites have the chemical formula ABO3, where A is one or more monovalent, divalent or trivalent elements and B is one or more pentavalent, tetravalent, trivalent or divalent elements.
It has been discovered that current methods of forming thin films of perovskite materials generally do not preserve the beneficial properties that these materials exhibit in bulk ceramic form. In particular, the dielectric constants of current thin films of these materials do not approach those of perovskite materials fabricated as bulk ceramics. In addition, if these materials are to be used in the thin film capacitors which make up memory devices, they must also exhibit small leakage current at high electric fields and have small loss tangents. The electrical, chemical, and mechanical properties must be fairly uniform over the operating temperature range of the device.
Preserving the high dielectric constants exhibited by the bulk ceramic forms of perovskite materials in thin film form is difficult. It has been discovered that the dielectric constant of these materials generally decreases with decreasing grain size. The grain size in bulk ceramics is generally 1 to 20 micrometers (xcexcm) while the grain size in a thin film is usually similar to the film thickness, generally 0.02 to 0.20 xcexcm. For example, the range of dielectric constants observed in bulk ceramic barium titanate (BaTiO3, hereafter referred to as BT) or barium strontium titanate ((Ba,Sr)TiO3, hereafter referred to as BST) is generally 1000 to 20,000 while the range of dielectric constants observed in thin films of these materials is only 100 to 600.
Much of the past research in this area has endeavored to preserve the high dielectric constant of perovskites such as BT and BST in thin film form. It is known that depositing thin films in a way that maximizes the grain size in the film serves to maximize the dielectric constant. This can be done by keeping the temperature of the substrate onto which the perovskite film is deposited at a high temperature, because higher deposition temperatures usually produce larger grain sizes in the deposited film. Higher substrate temperatures, however, may cause damage to existing devices and structures already formed on the substrate. Generally, temperatures should be kept as low as possible. Current methods of depositing thin film perovskites with high dielectric constants are thus limited by the film thickness and the potential damage caused by elevating the substrate to high temperatures.
The invention described is a method of forming an improved dielectric material by adding lead to an original perovskite material having an original critical grain size to form a lead enhanced perovskite material, then forming a layer of the lead enhanced perovskite material having an average grain size less than the original critical grain size whereby the dielectric constant of the layer is substantially greater than the dielectric constant of the original perovskite material with an average grain size similar to the average grain size of the layer. The average grain size of the layer so produced may be, for example, one-half or one-fourth of the original critical grain size. The critical grain size, as used herein, means the largest grain size such that the dielectric constant decreases with decreasing grain sizes when measured at the intended operating temperature. Preferably, the lead enhanced perovskite material is further doped with one or more acceptor dopants whereby the resistivity is substantially increased and/or the loss tangent is substantially decreased. Preferably, the original perovskite material has a chemical composition ABO3, where A is one or more monovalent, divalent or trivalent elements, and B is one or more pentavalent, tetravalent, trivalent or divalent elements.
Structures containing this improved dielectric material include a layer of lead enhanced perovskite material with average grain size less than the original critical grain size formed on the surface of a substrate. Other structures include such a layer of lead enhanced perovskite material interposed between two electrically conducting layers.
The applications of this invention are many. The materials presented may find use in many structures used in semiconductor circuitry, such as capacitors, MOS transistors, pixels for electro-magnetic radiation detecting arrays, and electrooptic applications. Devices which exploit the piezoelectric properties of many of these materals will benefit from this invention.
The advantages of this invention include substantially increased dielectric constants for perovskite materials formed with grain sizes typically found in thin films. In addition, the resistivity is generally increased and the loss tangent is generally decreased by the methods presented. The invention also provides for improved uniformity of the dielectric constant with respect to temperature for perovskite materials in thin film form.